Working through my options

I'm a pro photographer from Sydney but have never taken gear in the water before. I'm in the water nearly every day and just feel the need to get my gear wet, I haven't figured out what kind of photos I'll be taking, but I just want to have fun, and have something my kids can use.

I shoot Canon and have tons of spare gear, but I'm not really keen on housing for a DSLR due to bulk. A water proof compact is out of the question as I'd puke when I see the image quality they produce.

So I guess large sensor compact or mirror less is the go. Sony seem to be making great sensors these days, so that's a direction I'm seriously leaning.

Can anyone recommend a good large sensor mirrorless/compact water housing combo?

If you have never shot underwater before, my guess is that your biggest mental adjustment will be about lighting. You have to have external lights, and their diffusion and reach behave very differently under water than on land. Good underwater lights (strobes or video) are also a lot more expensive than land photographers think they should be. Many beginners start out cheap and often with only one light, but eventually upgrade. I would advise starting with one good strobe, experimenting, then see how you like the results.

Either the Sony nex cameras or Olympus 4/3, there are many more lenses available for the micro 4/3 systems though so think about what sort of pictures you want to take. Personally I would opt for one of the Olympus OMD in a nautical housing. Depends on your budget though.

The OMD in a Nauticam housing is neither particularly cheap nor particularly less bulky underwater than a housed canon DSLR. Since the original poster already has the camera and the lenses, getting the ports and housing for a Canon DSLR might be more economical than starting with a whole new m4/3 system.

The alternative I think if a housed DSLR is too much size-wise is a Sony RX100 system. But the devil is in the details -- to get nice pictures underwater you need strobes and arm, and wet lenses for wide angle and macro, and floats to make your system neutrally boyant. By the time you add all those things to your small RX100/Nauticam housing, it's not going to be a whole lot smaller than a DSLR

I'd start with an RX100 (mark I or mark II) in a Nauticam housing, a single handle, a wide angle wet lens and a single strobe. If you want to take wide angle photos that is. If you prefer macro, a canon G15/G16 might be a better path.

I've seen amazing work done with both systems. So maybe start out by browsing some underwater photos and figure out which ones you like/what you'd like to reproduce. Then you can research a system that allows you to do that

Are one time use underwater film cameras still being sold? It is very, very easy to trash camera (regardless of type) gear by flooding - all underwater gear requires extreme (should be read with emphasis) attention to maintenance. There are many reports of flooding on this site, even when people are being careful.

Olympus E-PL5 with their housing (PT-EP10) to keep cost down. This gives you the same sensor as the OMD (which is damn nice, and hard to beat excepting full frame) - better than the Sony (much smaller sensor) stuff for sure. Add your favorite strobe (my pick would be either the Sea&Sea YS-D1, or the YS-250).

Good news here is you can build on this system (great optical glass domes for fisheye, macro, etc.) AND get quality results...